This invention relates to methods for preparing anodes, that is negative electrodes, for use within secondary electrochemical cells. More particularly, it relates to the preparation of lithium-aluminum alloy for use in such electrodes. Electrochemical cells employing electrodes of this type are being developed to provide high current and capacity densities, high specific energy and power and long cycle life. These cells have application for battery power sources in electric or hybrid automobiles or for the storage of electric power generated by electric utility during periods of off-peak usage.
A substantial amount of work has been done in the development of high-temperature electrochemical cells and associated electrodes for meeting these applications. The cells showing the most promise employ alkali-metal negative electrodes, such as lithium or sodium or alloys including these metals, molten-salt electrolytes containing the alkali metal ions, and positive electrode materials synthesized from the chalcogen and halogen group elements. Possible materials for use in the positive electrode include not only reactants in elemental form but also compounds such as FeS.sub.2 and FeS. Examples of such secondary cells and their various components are disclosed in U.S. Pats. No. 3,716,409 to Cairns et al. entitled "Cathodes for Secondary Electrochemical Power Producing Cells," Feb. 13, 1973; 3,666,560 to Cairns et al. entitled "Electrochemical Power Producing Cell," May 30, 1972; and 3,488,221 to Hiroshi Shimotake et al., Jan. 6, 1970; as well as U.S. Pat. No. 3,907,589 to Gay and Martino entitled "Cathode for a Secondary Electrochemical Cell," Sept. 23, 1975; all assigned to the assignee of the present application.
Investigations of lithium and lithium-aluminum alloys have revealed that these materials offer promise as negative-electrode reactants within a high-energy cell. A lithium metal negative electrode has a higher chemical activity than the alloy and will accordingly produce a slightly higher electrical potential. Elemental lithium is molten much below the 350.degree.-450.degree.C. temperature range where the presently considered electrochemical cells are most often operated. Negative electrodes employing elemental lithium have been prepared with matrices of steel, carbon or other inert material impregnated with the molten metal. Even so, molten elemental lithium tends to excape from the negative-electrode structure and form direct paths for electronic conduction between the electrodes.
The use of solid lithium-aluminum alloy has been suggested to avoid the containment problems associated with molten lithium. However, electrodes of lithium-aluminum alloy offer an electrical potential of about three tenths of a volt, depending on lithium concentration, less than the elemental lithium electrodes. Also, the ampere-hour capacities and the maximum charge and discharge rates per unit volume of lithium-aluminum electrode have generally fallen below those obtainable with the elemental lithium electrodes. These shortcomings of the lithium-aluminum electrode in respect to the elemental lithium electrode can be minimized by employing the maximum practical composition of lithium that can be maintained within a solid alloy.
Previous development efforts on the lithium-aluminum alloy electrode have been based primarily on electrochemically depositing lithium onto a substrate of aluminum metal. After formation it has been necessary to precondition the electrode by initially operating through a number of cycles of slow charge and discharge. When the initial cycles are carried out too rapidly, regions of liquid metal alloy, resulting in pitting of the electrode, can be produced. (For example, see U.S. Pat. No. 3,445,288 to Buzzelli, "Aluminum Anode Electrical Energy Storage Device," assigned to Standard Oil Company, Cleveland, Ohio, May 1969.)
In an effort to minimize or eliminate the need for the lengthy preconditioning steps, other workers investigated the use of a porous aluminum substrate for receiving the lithium electrodeposition. Efforts of this type are described by Walsh et al. in U.S. patent application Ser. No. 416,311, "Modular Electrochemical Cell," filed Nov. 15, 1973, now U.S. Pat. No. 3,887,396 assigned to the assignee of the present application; and in "High Performance Batteries for Off Peak Energy Storage," Progress Report for the period January - June 1973, ANL-8038, March 1974. In these latter efforts various commercially available, porous aluminum plaques were employed as substrates. The substrates included compacted fiber disks, wire disks, aluminum foam, and sintered aluminum powder plaques. The lithium-aluminum alloy formation involved electrically discharging and charging a Li(Al)/Li cell (about 0.3 volt) either at gradually increasing currents or at a constant current. At least two or three lengthy cell cycles, about 10-40 hours each depending on size, were required in which lithium was deposited onto the aluminum substrate during the discharge, and lithium was removed from the alloy during the cell charge. Formation cells of this type are operated at a sufficiently high temperature, e.g. 400.degree.-450.degree.C., to maintain the electrolyte of LiCl/KCl eutectic salt in a molten state. Ordinarily this technique is successful in preparing lithium-aluminum alloys having 40 to 45 but no more than about 50 atom percent lithium. Alloys greater than 50 atom percent lithium involve substantial risk of molten alloy formation with the resulting loss of reactant and degradation of the substrate.
Another problem in the use of electrochemically prepared lithium-aluminum electrodes is in dimensional stability. The aluminum substrates have been found to expand during the alloy formation and preconditioning steps. The expansion is attributed to excessive lithium influx into the substrate interstices and to nonuniform electrodeposition and reaction of lithium throughout the substrate.
In the preparation of lithium-aluminum alloys by metallurgical techniques, care must be taken to avoide loss of lithium by evaporation or its contamination by chemical reaction at the elevated temperatures. It can be seen by examining the binary phase diagram of the lithium-aluminum system shown in the figure that temperatures of up to 718.degree.C., for 50-50 atom percent lithium-aluminum, are required to produce a completely molten mixture. At these temperatures elemental lithium is quite reactive and exerts a significant vapor pressure.
On solidifying a molten mixture of most lithium and aluminum compositions in excess of 50 atom percent lithium, a two-phase mixture of liquid and solid is first formed. The temperature for a particular composition at which freezing begins on cooling and melting ends on heating is termed the liquidus temperature. Similarly the temperature at which freezing ends on cooling and melting begins on heating is termed the solidus temperature. These temperatures are represented by solid lines in the equilibrium phase diagram of the FIGURE, with the letter L used to signify regions in which the liquid phase is present at equilibrium. For concentrations between about 50 and 70 atom percent lithium, a two-phase region of liquid and solid is represented by the spherical triangle above 522.degree.C. in the FIGURE labeled as including liquid and beta (.beta.) particles. For concentrations in excess of about 56 percent lithium, cooling can continue to 522.degree.C. without complete solidification. On further cooling, particles of the lithium-aluminum beta (.beta.) composition react with the lithium-rich liquid to form Li.sub.3 Al.sub. 2. This lithium-rich compound will often coat the solid particles and thereby isolate domains of aluminum-rich composition out of contact with the liquid. Consequently, the solidified mixture can be heterogeneous with regions of beta composition disbursed throughout regions having elevated lithium concentrations. On using a nonuniform lithium-aluminum alloy of this type as an electrode within an electrochemical cell operating at 350.degree.C. to 450.degree.C., lithium-rich portions of the electrode can become molten and be lost into the electrolyte.
It was previously believed that the lithium-aluminum compound formed at the peritectic temperature of 522.degree.C. was Li.sub. 2 Al rather than Li.sub. 3 Al.sub. 2. (Investigators have determined that the lithium-aluminum compound having approximately twice the atomic concentration of lithium as aluminum is more accurately designated Li.sub. 9 Al.sub. 4. For the purpose of this application, this compound shall continue to be designated Li.sub. 2 Al.) Many lithium-aluminum phase diagrams found in the literature will show a region of solid alloy mixture of beta and Li.sub. 2 Al rather than Li.sub. 3 Al.sub. 2 up to 522.degree.C. This region extends to nearly 70 atom percent lithium (see Hansen and Anderko, Constitution of Binary Alloys, page 1, McGraw-Hill, 1958).
The lithium-aluminum phase, Li.sub. 3 Al.sub. 2, was discovered only a short time ago (see Tebbe et al., "Eine Neue Phase Im System Li/Al", Zeifschr. F. Naturf. Tl 6, 286, pp. 600-605, 1973). Taking this new phase into account, the temperature-composition parameters within the region of 56-70 atom percent lithium are believed to be more accurately represented by the diagram of the FIGURE than those previously given in the literature. As can be seen from the FIGURE, completely solid mixtures of lithium-aluminum phases do not exist above 60 atom percent lithium within the desired cell operating temperature range of 350.degree.-450.degree.C. As shown in the FIGURE, the solidus temperature for lithium compositions of above 60 to nearly 70 atom percent is about 320.degree.C. Accordingly, the inventors have devised new methods of preparing negative electrodes to take these discoveries into account.
Therefore, in view of the above, it is an object of the present invention to provide a method of preparing an essentially uniform alloy of lithium and aluminum that will remain solid at the cell-operating temperature for use as an electrode within a high-temperature electrochemical cell of high specific energy and capacity.
It is also an object to provide a method of preparing a solid lithium-aluminum electrode having in excess of 50 atom percent lithium without employing lengthy electrochemical processes.
It is a further object to provide an essentially uniform and dimensionally stable electrode composition of about 50-60 atom percent lithium and 40-50 atom percent aluminum.